Get rid of tail heavy physics

[Ex-jazz]

I understood so, the plane without tail will violently pitch up, because the NP suddenly move front of the CG.

[Dream Child]

If a picture is worth a thousand words, here are two thousand words.  First images I thought of when i read the initial post.

Flak hit from below, forcibly removes the A20's tail.  You can see the pieces flying back.  Based on getting hit from below and the arguments above, the nose should have pitched down.  Sure looks like he pitched up.  Not making light of what may  be photos of two aircrew getting killed, but there is no doubt the nose pitched up as the initial photo and the following were taken from the same plane and the first shows the trailing A20 from slightly above.

If you had lost just the stabilizers, this would be a valid argument. Losing body parts changes the center of lift more than what we're talking about here, and puts the stabilization point in front of the center of gravity.

[Stoney]

If you had lost just the stabilizers, this would be a valid argument. Losing body parts changes the center of lift more than what we're talking about here, and puts the stabilization point in front of the center of gravity.

You obviously didn't read the link I posted...

[Dream Child]

Moot jogged my memory.  dTango posted this link from an earlier thread.  This explains the math behind the issue:

http://adg.stanford.edu/aa241/stability/staticstability.html

This is really funny. Look at the equation and you see the lift produced by the tail is negative. In other words, it's pushing down on the tail of the airplane, kinda like what I said in the first place.

[Dream Child]

You obviously didn't read the link I posted...

Obviously, I did...

[Stoney]

This is really funny. Look at the equation and you see the lift produced by the tail is negative. In other words, it's pushing down on the tail of the airplane, kinda like what I said in the first place.

[sigh]...

Which equation from that link are you referencing?

[BnZs]

In order to illustrate what the horizontal stabilizer actually does for the plane, I'll explain how the elevator works, from an aerodynamic perspective.  During flight, the horizontal stabilizer, as BnZ points out, provides pitch stability.  It creates the necessary lift to maintain the pilot's desired AoA on the wing.  When the elevator (attached to the horiz stab) is deflected up (pilot pulls back on the stick), it decambers the horizontal stabilizer, decreasing the lift created, reducing the moment applied at the tail, and allowing the nose to pitch up.  When the elevator is deflected down, it adds camber to the horizontal stabilizer, increasing the lift created, and forcing the nose to pitch down.

BnZ, there is a difference between net pitching moment for the entire aircraft, and the airfoil pitching moment.

Okay, I'm interested. If the CG is forward AND the airfoil itself wants to rotate down, what is making the aircraft want to pitch up, besides the forces of the horizontal stab?

[Stoney]

Okay, I'm interested. If the CG is forward AND the airfoil itself wants to rotate down, what is making the aircraft want to pitch up, besides the forces of the horizontal stab?

Let's step back a moment (no pun intended)...

Think about a symetrical airfoil.  A symetrical airfoil has no pitching moment through its entire range of useable AoA.  Its design lift coefficient = 0, in that it produces zero lift at 0 degrees AoA.  However, at any positive AoA up to but not including the stall AoA, a symetrical airfoil will produce lift, but will do so without producing any pitching moment.  So, for example, at 4 degrees AoA on a plane with a symetrical airfoil, what other moments exist that make the plane want to pitch up?  What other moments exist that make the plane want to pitch down?

Notice this graph that is in the link that dTango provided.  What stands out when comparing the pitching moments of the different parts of the aircraft? (Cm = pitching moment).

[Ex-jazz]

This is really funny. Look at the equation and you see the lift produced by the tail is negative. In other words, it's pushing down on the tail of the airplane, kinda like what I said in the first place.

With horizontal stabilizer = NP(lift) is behind of the CG = pitching down.


Without horizontal stabilizer = NP(lift) is front of the CG = pitching up.



If NP(lift) is front of the CG = pitching up

[Baumer]

I may be over simplifying this a bit but, if the aerodynamic center (for the wing), is forward of the the center of gravity the nose should pitch up.



I believe that for most of the aircraft in AH, we are not talking about symmetrical airfoils either, so the discussion of aerodynamic center can get rather more complicated.

[hitech]

Baumer: Nice diagram. What the OP fails to realize is that with pictures describing Center of lift and CG. The Center of lift is normally shown as only the net lift of both tail and wing.

Flying with an down force on the tail simply creates more drag, and hence is not normally desired characteristic for fighters .

HiTech

[Wedge1126]

Baumer: Nice diagram. What the OP fails to realize is that with pictures describing Center of lift and CG. The Center of lift is normally shown as only the net lift of both tail and wing.

I'm assuming this is done with the elevators in a neutral position?

[PJ_Godzilla]

Umm...no, not normally. It's a factor of center of gravity vs center of lift. If I have to push down on the tail to keep the nose of the plane up, and then I lose that push, the nose goes down. While the math is a bit messy, the concept is not.

Agreed, at least that's what we learned in Aero Eng.

What they're seeing is probably a dynamic effect. Modelled in level flight, I can't imagine a scenario where the airplane wouldn't pitch nose down after losing the horistab. THat's just because, in statically stable aircraft, anyway, the cg is forward of the Center of Pressure - the extent of which is labelled the static margin.

However, IRL, if you're already pitched nose down, you're doing so by creating lift on the elevator. Lose that and the nose will likely pop up.

Of course, even with the stab locked and power off, most a/c will exhibit classic phugoid (long period) motion, nosing up until lift abates from dropping KE, dropping the nose until lift recovers on both surfaces and causes pitch up. That's the nature of longitudinally stable a/c - self-recovering divergence. The short period motion has period proportional to the static margin to which you refer earlier (diff b/w NP and CG).

All I'm saying here, short form, is it depends on the force being created by the horistab at time of "separation event".

[Stoney]

Agreed, at least that's what we learned in Aero Eng.

What they're seeing is probably a dynamic effect. Modelled in level flight, I can't imagine a scenario where the airplane wouldn't pitch nose down after losing the horistab. THat's just because, in statically stable aircraft, anyway, the cg is forward of the Center of Pressure - the extent of which is labelled the static margin.

However, IRL, if you're already pitched nose down, you're doing so by creating lift on the elevator. Lose that and the nose will likely pop up.

Of course, even with the stab locked and power off, most a/c will exhibit classic phugoid (long period) motion, nosing up until lift abates from dropping KE, dropping the nose until lift recovers on both surfaces and causes pitch up. That's the nature of longitudinally stable a/c - self-recovering divergence. The short period motion has period proportional to the static margin to which you refer earlier (diff b/w NP and CG).

All I'm saying here, short form, is it depends on the force being created by the horistab at time of "separation event".

Ok, think about this.  Most horizontal stab airfoils are symmetrical.  They are also placed on the aircraft at a positive angle of incidence, relative the wing, typically at an angle that will minimize trim drag at the design lift coefficient for the wing.  So even when the elevator is not deflected at all, the horizontal stabilizer is producing lift.  Why, if the aircraft naturally has a nose down pitching tendency, do we decrease the amount of lift the H-stab produces in order to pitch the nose of the aircraft up?  If the H-stab provides a "down" force, we would need to increase the amount of lift it produces in order to pitch the nose up.  In actuality, we increase lift on the H-stab to pitch the nose down, and decrease the lift on the H-stab in order to pitch the nose up.  

[EDIT] Center of pressure is not the same thing as Aerodynamic center.  The static margin equation uses Aerodynamic Center, not Center of Pressure.

[Ex-jazz]

Ok, think about this.  Most horizontal stab airfoils are symmetrical.  They are also placed on the aircraft at a positive angle of incidence, relative the wing, typically at an angle that will minimize trim drag at the design lift coefficient for the wing.  So even when the elevator is not deflected at all, the horizontal stabilizer is producing lift.  Why, if the aircraft naturally has a nose down pitching tendency, do we decrease the amount of lift the H-stab produces in order to pitch the nose of the aircraft up?  If the H-stab provides a "down" force, we would need to increase the amount of lift it produces in order to pitch the nose up.  In actuality, we increase lift on the H-stab to pitch the nose down, and decrease the lift on the H-stab in order to pitch the nose up.  

[EDIT] Center of pressure is not the same thing as Aerodynamic center.  The static margin equation uses Aerodynamic Center, not Center of Pressure.

Now I am a bit confused... This physics make my head hurt.

I though the planes, with traditional wing-tail configuration, have a horizontal stabilizer with negative angle of incidence to provide a negative lift for the Longitudinal stability.